This invention relates to a test and measurement instrument with digital storage.
FIG. 1 of the accompanying drawings illustrates in simplified fashion the overall architecture of an optical time domain reflectometer (OTDR) 2, used to test the condition of an optical fiber 6. FIG. 1 is not intended to illustrate a specific prior art OTDR, but to provide a context in which features that are currently found in OTDRs can be discussed. OTDR 2 comprises a laser diode 8 that is energized intermittently by a laser driver 10 to launch interrogation pulses into fiber 6 by way of a directional coupler 14 and a launch fiber 16, which is connected to fiber 6 by a connector 18. OTDR 2 receives return light from fiber 6 due to Rayleigh backscattering and Fresnel reflection. The intensity of the backscattered and reflected light depends on the condition of the fiber under test.
A portion of the return light received from fiber 6 is coupled through coupler 14 to a photodetector 20, which generates a current signal representative of the intensity of the return light. The current signal is converted to a voltage signal and the voltage signal amplified by a transimpedance amplifier (not shown), and the amplified voltage signal provided by the transimpedance amplifier is sampled and converted to digital form by an analog-to-digital converter (ADC) 24. A timing controller 30 controls the timing of the operation of ADC 24 relative to laser driver 10.
The digital sample values V provided by ADC 24 are written into an acquisition memory 28 at respective addresses that depend on the timing T of the samples relative to the respective interrogation pulses. For the purposes of this description, it will be assumed that memory 28 has 2048 storage locations.
The interval between launch of an interrogation pulse and sampling of the output of the photodetector determines the location of the length segment of the fiber from which the return light is received. Timing controller 30 operates in response to signals provided by user interface 40 to establish the effective sampling frequency of the return signal and the interval relative to the interrogation pulses within which sampling takes place. In this manner, the condition of fiber 6 within an acquisition window is tested and the user controls the length and position of the acquisition window and the resolution with which the fiber is tested. The data record V (T) stored in memory 28 represents the variation in condition of the fiber under test as a function of position within the acquisition window.
OTDR 2 also comprises a cathode ray tube (CRT) display device 32 having a rectangular array of 512.times.512 addressable pixels. Display device 32 is used to provide a display in a rectangular cartesian coordinate system of intensity of return light received by detector 20, represented by voltage, as a function of distance along the fiber under test, represented by time. A display controller 34 provides deflection signals to display device 32. The deflection signals cause the electron beam of the display device to execute a horizontal raster scan. Each addressable pixel of display device 32 is associated with a unique time slot within the field of the raster.
In order to provide a display, a segment of the address space of acquisition memory 28 is selected by use of horizontal position and expansion signals provided by user interface 40 and data values representative of the contents of the selected segment of acquisition memory 28 are loaded into a display memory 36. Display memory 36 has 512.times.512 addressable memory locations, each of which can store a single bit. The addressable memory locations of display memory 36 correspond on a one to one basis with the raster time slots of the addressable pixels of display device 32. Display controller 34 generates 512 equally spaced address words T within the selected segment of the address space of memory 28, and reads the associated data values V from the acquisition memory. Display controller 34 translates each data value V and its associated address word T into the raster time slot of the pixel that should be illuminated in order to display a data point at the proper horizontal and vertical position on the screen of display device 32, and writes a logical one into the corresponding memory location of display memory 36. When display memory 36 has been loaded, display controller 34 repetitively scans the addressable pixels of display device 32 and repetitively reads the contents of display memory 36 in timed relation to the scanning of display device 32. The digital values read from memory 36 are converted to analog form and are applied to the intensity control input of display device 32, and an image of the contents of memory 36 is formed. In this fashion, a graphic display is provided of the portion of the data record defined by the horizontal position and expansion signals and this display represents variation of the condition of the fiber as a function of distance within a display window.
Since the data record contains 2,048 sample values and only 512 equally spaced values are used to create the display, the resolution of the display may be altered. For example, the condition of the fiber over the entire acquisition window may be displayed at low resolution by reading every fourth sample from the first to the 2,045th, whereas maximum resolution over a shorter display window is achieved by displaying a block of 512 adjacent samples. In order to increase the resolution still further, it is necessary to acquire new data at a higher effective sampling frequency. When a new data record is acquired, the original data record is overwritten.